Conventionally, in a disc apparatus, such as a CD player for reproducing a compact disc (DC) having concentric tracks in which digital audio data are recorded as a train of pits, a laser beam is irradiated along a track on the disc driven by a spindle motor at a constant linear velocity (CLV), and the digital audio data are reproduced by detecting changes in the intensity of the reflected light caused by the presence or absence of the pits.
Although the bit error rate during data reproduction may reach a value in an order of 10.sup.-5, an error correcting operation is performed with the acid of error detection and error correction codes to obviate any inconveniences which might otherwise arise under usual operating environments.
Meanwhile, in a CD player provided with an optical reproducing head, it is a frequent occurrence that the servo systems such as the focusing servo or tracking servo for the reproducing head is out of order so that regular data playback cannot be achieved. In such case, error correction cannot be made even with the use of the error detection or error correction code, so that data playback is discontinued transiently.
In a car-laden or portable CD player, which may be subjected to extremely large impacts or vibrations in a manner different from a stationary CD player for domestic use, a mechanical vibration proofing system is provided to prevent the servo system from falling into disorder in the above described manner.
On the other hand, in a so-called CD-I(CD-Interactive) system in which video data and letter or character data are recorded simultaneously on the compact disc (CD) in addition to the audio information, seven modes as shown in FIG. 1 are standardized as audio information.
In the CD-DA mode in which the sound quality level corresponds to the current 16-bit PCM, linear pulse code modulation (PCM) with the sampling frequency of 44.1 Khz and the number of bits of quantization equal to 16, is employed. In the A level stereo mode and the A level monaural mode having the sound quality corresponding to the long-playing record, adaptive differential pulse code modulation (ADPCM) with the sampling frequency of 37.8 kHz and the number of bids of quantization equal to 8, is employed. In the B level stereo mode and the B level monaural mode having the sound quality corresponding to FM broadcasting, ADPCM with the sampling frequency of 37.8 kHz and the number of bits of quantization equal to 4, employed. Finally, in the C level stereo mode and the C level monaural mode, having the sound quality corresponding to the AM broadcasting, ADPCM with the sampling frequency of 18.9 Khz and the number of bits of quantization equal to 4, is employed.
That is, turning to FIG. 1, in the A level stereo mode, as contrasted to the CD-DA mode, the bit reduction ratio is 1/2 and data are recorded at every two sectors (represents a sector where data recording in made) with the reproducing time for a disc being about two hours. In the A level monaural mode, the bit reduction rate is 1/4 and data are recorded at every four sectors, with the reproducing time being about four hours. In the B level monaural mode, the bit reduction ratio is 3/8 and data are recorded at every eight sectors, with the reproducing time being about eight hours. In the C level stereo mode, the bit reduction ratio is 1/8 and data are recorded at every eight sectors, with the reproducing time being about eight hours. In the C level monaural mode, the bit reduction ratio is 1/16 and data are recorded at every sixteen sectors, with the reproducing time being about sixteen hours.
Heretofore, the rotational velocity of the disc in each of the above modes is the same, that is, the transfer rate of recordable data per second on the transfer rate of reproducible data per second is 75 sectors. When recording audio data on a disc in the B level stereo mode, for example, the data transfer rate in the B level stereo mode is 18.75 (75.div.4) sectors/second, audio data are discretely recorded at every four sectors, from the first sector of the innermost track towards the outermost track on the sector-by-sector basis and, after audio data are recorded on the outermost track, audio data are again recorded at every four sectors from the second sector of the innermost track towards the outermost track. That is, audio data are recorded on the disc from the innermost track towards the outermost track, from the innermost track towards the outermost track, from the innermost track towards the outermost track and from the innermost track towards the outermost track. Thus, during reproduction, data are not reproduced when the reproducing head jumps (or reverts) from the outermost track towards the innermost track, so that reproduction of a piece of music is discontinued.
There has hitherto been provided a disc recording apparatus adapted for recording digital data conforming to the above described CD or CD-I standard on a write-once optical disc or overwrite type magneto-optical disc. However, with such disc recording apparatus, the servo system for focusing servo or tracking servo of the recording head tends to be disengaged or out of order due to mechanical disturbances, such as vibrations or impacts, such that recording is discontinued transiently.
In view of the above described status of the conventional disc apparatus, it is an object of the present invention to provide a data recording method in which the rotational velocity of the disc remains the same for each mode, in which data may be recorded continuously on a track of the disc which is rotated at a rotational velocity faster than the rotational velocity corresponding to the data transfer rate, when the rotational velocity of the disc is faster than the rotational velocity corresponding to the data transfer rate, as in data recording in the B level stereo mode, and in which continuous data reproduction may be made at the time of data reproduction. It is another object of the present invention to provide a data recording method in which data may be recorded continuously on a recording track of the disc-shaped recording medium even though the servo system is in trouble due to disturbances.
It is a further object of the present invention to provide a data reproducing method in which data continuously recorded on a track of a disc rotated at a rotational velocity faster than the rotational velocity corresponding to the data transfer rate may be reproduced at a predetermined data transfer rate. It is a further object of the present invention to provide a data reproducing method in which data may be continuously reproduced from a recording track on a disc-shaped recording medium even though the servo system is in trouble due to disturbances.